Neoplastic transformation produces many changes in cell physiology. We have utilized morphologic techniques to study these changes and to understand basic cellular mechanisms of organelle movement, function and compartmentalization of intracellular materials. We have employed ultrastructural immunocytochemistry, single cell microinjection, image intensification microscopy, and biochemical preparative techniques to follow substances from the outside of the cell to the interior. The study of the pathway of receptor-mediated endocytosis and Golgi traffic by these methods has revealed: (1) Epidermal growth factor (EGF) clusters in coated pits of the Golgi prior to lysosomal delivery. (2) Adenovirus enters cells through the same pathway, and coincubation of toxic conjugates of EGF and Pseudomonas toxin with adenovirus leads to a 10,000-fold enhancement of toxicity, apparently through and adenovirus-mediated lysis of receptosomes. (3) Microinjection of a monoclonal antibody to tubulin decorates microtubules in living cells and blocks saltatory movement of organelles such as receptosomes and lysosomes. In other studies, it was shown that an antibody to Harvey MSV p21src reacts with a normal cell protein in human epithelial cell lines in culture.